Are you asking about the bullpups that Mossberg sold? Those kits have been out of production for a while. I've read that 'Havlin Gun Sales' were the last one to sell the kits. I have no idea who Havlin Gun Sale is but there is a website that someone by that name that specializes in Mossberg stuff.

Have you ever handled one? They feel different than your usual shotgun. Kinda awkward. I know that some people really like them though.